This study explores DXA's utility as a new reference method for body composition, and compares its total and regional tissue masses in different athletic groups and controls. Findings: Body Composition. Athletes had significantly greater lean tissue and less fat than controls, but bone mineral content (BMC) depended on the type of activity. Anthropometry proved superior to BIA in predicting fat and fat-free mass in male athletes, and the accuracy of predictions of published skinfold equations applied to the athletes appeared to depend upon appropriate skinfold selection. DXA data showed athletes appeared to distribute fat differently from sedentary controls after controlling for age and adiposity, although this was not influenced by exercise type or differences in lean tissue distribution. Female athletes had different distributions of bone, lean and fat tissue to anorexics and controls. A new body composition descriptor - the DXA morphotype - was derived for athletic groups using bone and soft tissue Z scores, which discriminated the groups differently from traditional somatotype measures. Differences in fat content by MRI and DXA of the upper leg were consistent with the variable lipid fraction of adipose tissue with adiposity; while MRI muscle mass was slightly better predicted from anthropometry than DXA lean tissue mass. Findings: Bone. Eleven out of 26 female athletes had significantly reduced bone mineral density (BMD) at the lumbar spine, which was best predicted from the combined index (CX); the sum of BMI and oestrogen status (OS, defined as 0 for oligo- or amenorrhoea; 1 for eumenorrhoea or supplemented). These data suggest women athletes with a CX ≤ 20.4 carry a 2.4 fold increased risk of spinal osteopenia. Variation in BMD between male athletic groups exposed differences in physical impact. Rugby showed the highest BMD of all groups, after correction for body size. Compared with controls, running was associated with increased BMD in the legs, while cycling was associated with a decreased BMD in the spine. Cyclists had a mean T score of -1.16, and were seven times as likely as controls to have osteopenia. While unimportant in the legs, muscular torque appeared to contribute to increased BMD in the arms and spine, suggesting different mechanisms for adjusting skeletal architecture may operate at different sites.